The present disclosure generally relates to stimulation of subterranean formations, and, more specifically, to methods for complexing metal ions in a subterranean formation with a perfluorinated chelating agent to decrease the occurrence of precipitation in the subterranean formation.
Treatment fluids can be used in a variety of subterranean treatment operations. Such treatment operations can include, without limitation, drilling operations, stimulation operations, production operations, remediation operations, sand control treatments, and the like. As used herein, the terms “treat,” “treatment,” “treating,” and grammatical equivalents thereof refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. Use of these terms does not imply any particular action by the treatment fluid or a component thereof, unless otherwise specified herein. More specific examples of illustrative treatment operations can include drilling operations, fracturing operations, gravel packing operations, acidizing operations, scale dissolution and removal operations, sand control operations, consolidation operations, and the like.
Downhole acidizing operations and other dissolution operations may be used to stimulate a subterranean formation to increase production of a hydrocarbon resource therefrom. During an acidizing operation or a like dissolution operation, an acid-soluble material in the subterranean formation may be dissolved by one or more acids to expand existing flow pathways in the subterranean formation, to create new flow pathways in the subterranean formation, and/or to remove acid-soluble precipitation damage in the subterranean formation, thereby stimulating the formation's production capabilities. Introduction of an acidizing fluid to a subterranean formation may take place at matrix flow rates without fracturing of the formation matrix or at higher injection rates and pressures to fracture the formation. The acid-soluble material being dissolved by the acid(s) may be part of or formed from the native formation matrix or have been deliberately introduced into the subterranean formation in conjunction with a stimulation or like treatment operation (e.g., bridging agents, proppants, or gravel particulates). Illustrative substances within the native formation matrix that may be dissolved by an acid include, but are not limited to, carbonates, silicates and aluminosilicates, which may be present alone or in combination with one another in formations of mixed mineralogy. Other substances may also be dissolved during the course of performing an acidizing operation, and the foregoing substances should not be considered to limit the scope of substances that may undergo acidization.
Carbonate formations can contain minerals that comprise a carbonate anion (e.g., calcite (calcium carbonate), dolomite (calcium magnesium carbonate), and siderite (iron carbonate)). When acidizing a carbonate formation, the acidity of the treatment fluid alone may often be sufficient to solubilize the carbonate material by decomposing the carbonate anion to carbon dioxide and leeching a metal ion into the treatment fluid. Both mineral acids (e.g., hydrochloric acid) and organic acids (e.g., acetic and formic acids) may be used to treat a carbonate formation, often with similar degrees of success. Since it is relatively inexpensive, hydrochloric acid is very commonly used, typically in concentrations up to about 28% by volume.
Siliceous formations can include minerals such as, for example, zeolites, clays, and feldspars. As used herein, the term “siliceous” refers to a substance having the characteristics of silica, including silicates and/or aluminosilicates. Dissolution of siliceous materials through acidization is thought to be considerably different than acidizing carbonate materials, since the acids that can be effective for acidizing carbonate materials may have little effect on siliceous materials. In contrast, hydrofluoric acid, another mineral acid, can react very readily with siliceous materials to promote their dissolution. Oftentimes, a mineral acid or an organic acid can be used in conjunction with hydrofluoric acid to maintain a low pH state as the hydrofluoric acid becomes spent during dissolution of a siliceous material, where the low pH state helps promote continued solubilization of the siliceous material. Many types of siliceous formations can also contain varying amounts of carbonate materials. Most sandstone formations, for example, contain about 40% to about 98% sand quartz particles (i.e., silica), bonded together by various amounts of cementing materials, which may be siliceous in nature (e.g., aluminosilicates or other silicates) or non-siliceous in nature (e.g., carbonates, such as calcite). When siliceous materials are co-present with carbonate materials, significant precipitation issues can sometimes be encountered, as discussed below.
In some instances, it can be desirable to remove a carbonate material from a siliceous formation prior to acidizing the siliceous material therein to decrease the occurrence of precipitation in the subterranean formation. For example, calcium ions liberated from the carbonate material can react readily with fluoride ions from hydrofluoric acid to form highly insoluble calcium fluoride, which can often be more damaging to the subterranean formation than if the acidizing operation had not been performed in the first place. Different metal ions liberated from other carbonate materials can also be problematic in this regard. Metal ions liberated from aluminosilicates and other metal-containing siliceous materials may also be problematic in this regard.
Another approach that can be used to address the presence of metal ions in a subterranean formation is to employ chelating agents that effectively sequester any problematic metal ions in a metal-ligand complex, such that they are substantially unable to undergo a further reaction to produce calcium fluoride or other types of metal-containing precipitates. As used herein, the terms “complex,” “complexing,” “complexation” and other variants thereof refer to the formation of a metal-ligand bond. Although complexation of a metal ion may involve a chelation process in some embodiments, complexation is not deemed to be limited in this manner. Chelating agents may also directly dissolve a carbonate material without first liberating the metal ions therefrom, even in the absence of another acid, and function in a similar manner to mitigate the formation of metal-containing precipitates. Although precipitation can be a particular concern when acidizing a siliceous material, chelating agents may also be used with similar benefits in conjunction with acidizing subterranean formations that comprise substantially only a carbonate material by limiting the formation of carbonate scale. Likewise, complexation of metal ions liberated from siliceous materials may also be desirable.
One difficulty that may be encountered with chelating agents is that they usually exhibit a relatively limited pH range over which they may effectively complex metal ions to form a metal-ligand complex. Chelating agents comprising carboxylic acids, for example, may be inactive for metal-ion complexation at pH values below the pKa(s) of their carboxylic acid groups, since the carboxylic acid groups may be substantially protonated at such pH values and unable to donate a lone pair of electrons for forming a metal-ligand bond. For many common chelating agents comprising carboxylic acid groups, the pKa values of the carboxylic acids reside in the 3-5 range. Even the most acidic carboxylic acids in common chelating agents only reside in the 1.5-2.5 pKa range. Thus, many conventional chelating agents are believed to be inactive for metal ion complexation at the initial pH values of highly acidic acidizing fluids (e.g., a pH of less than about 2). Hence, metal ion complexation at low pH values remains a persistent issue that has not yet been effectively solved.